It would be possible to make a planetary hub with a closer ratio, but the 4:3 ratio is kinda optimal because the sun and planet gears are of equal diameter. Moving too far away from that ratio results in either a tiny sun, or tiny planet gears, that might not be as robust mechanically, or require a bigger ring gear.

Probably for this reason, the narrow range AC hub has two planetary mechanisms, which adds cost and complexity:

http://www.sturmey-archerheritage.co...s/pic-70.1.jpg